She knew Ali has a twin and they switched places
Jenna knows about Alison's secret twin. --A
His secret is that when he was younger he molested his step sister Jenna
Jenna knew the entire time that Ali had a twin named Courtney
The biggest secret they all share is they blinded a girl named Jenna Marshall.
That her step brother Toby was touching her in nasty places.
Ali caught him sexually abusing his step sister, Jenna.
No Ali is not being Jenna in pretty little liars.
Emily's secret is that she was a lesbian, but that gets out, so A just taunts her the Jenna Thing and the mystery of Alison.
Mona had already knew about the "Jenna thing" ever since that she never liked the Pretty Little Liars so she decided to get close to Hanna as a friend (Keep your friends close but your enemies closer)She had decided to team up with Jenna, Jenna had already new a little of the Girls Secret.
From what I have read I believe it is that she knows Courtney and Ali switched places. In the shows however it seems that she has something to do with Ali's murder.
its that they were with ali and ali tried to pul a prank on Toby by settin his tree house on fire but accidently it goes thru the win Dow and blinds jenna
Jenna really can see.